1. Field of the Invention
This invention relates generally to fibrous webs of a type suitable for use in absorbent articles such as disposable diapers, sanitary napkins, incontinency briefs, and the like, and to a method for making such fibrous webs.
2. Description of the Prior Art
In the art of absorbent articles such as disposable diapers, sanitary napkins, incontinency briefs, etc., it has been common practice to form the absorbent article as a laminated structure comprising a body of absorbent material such as air-felt, cellulosic fluff, or the like, disposed adjacently to a liner layer which on the side opposite the absorbent body is disposed against the wearer's skin. Accordingly, the layer contiguous to the skin must have a highly transmissive character as regards the flow of fluid, e.g., urine, menstrual fluid, etc., to the absorbent body and away from the skin of the user. Further, in addition to such wicking or fluid transmissivity function, the liner layer must have a skin-side surface which is soft and nonabrasive, i.e., gentle to the skin of the user. Further, such liner layer must have sufficient mechanical strength and flexibility to impart structural integrity and maintain the form of the absorbent article, due to the fact that the absorbent body itself typically has a low degree of mechanical strength and structural integrity.
Although the prior art has attempted to accommodate the foregoing requirements for the liner layer of the absorbent article, such efforts primarily have been directed at improving the mechanical strength and integrity characteristics of the liner material, frequently at the expense of its softness and surface texture characteristics. In some instances, the prior art has attempted to provide the liner as a laminated structure, wherein a top layer, for disposition against the skin of the wearer, is soft and fluffy in character, while a backing layer, disposed contiguous to the absorbent body, is relatively stronger and less flexible in nature Nonetheless, such composite structures, in the provision of the backing layer of such type, generally increase the resistance to fluid transmissivity through the liner to the absorbent body.
U.S. Pat. No. 4,333,979 to M. A. Sciaraffa, et al. discloses a nonwoven web of thermoplastic fibers which is pattern-bonded and further embossed to provide an increase effective thickness providing softness and bulk of the nonwoven material while retaining other desirable characteristics such as strength. The web is a spunbonded material composed of closely-spaced point fused areas (constituting a spunbonded pattern) with the subsequently applied embossing pattern comprising much larger embossments. The resultant material is said to be highly effective as a liner for disposable products such as diapers, sanitary napkins and the like. The nonwoven web in this system is bonded by passage through a pattern nip formed by heated rolls, whereby individual compacted fused areas are formed occupying about 5 to 50% of the total web area with a density of about 50 to 3,200 fused areas per square inch. The further processing involves application of a gross embossing pattern imparting a substantially permanent deformation to the web in the form of a pattern of depressed areas. This gross pattern embossment is preferably obtained by passing the pattern-bonded web through a nip formed by a matched set of heated web embossing rolls. The gross pattern occupies an area of about 5-80% of the web surfaced with embossed pattern frequency in the range of from about 1 to 500 depressions per square inch. The disclosed webs have a basis weight in the range of from 0.4 ounces per square yard to 2.0 ounces per square yard, with web density being in the range of 0.08 to 0.20 gm/cc. The material of the nonwoven web includes meltspun fibers of thermoplastics such as polyolefins, polyethylene, polypropylene, polyesters, polyamides and composites thereof with cellulosic fibers. The patent describes the use of the disclosed nonwoven web as a topsheet of a diaper including a backsheet, an absorbent layer, and topsheet.
The teachings of the Sciaraffa patent relate to a doubly-embossed, single layer web. Because the web is a monolayer, the dual embossing steps will provide strengthening of the web structure, but such improvement in structural integrity is obtained at the expense of the softness and flexibility characteristics of the web stock from which the embossed product is made.
U.S. Pat. No. 4,374,888 to S. R. Bornslaeger discloses a non-woven fabric laminate suitable for use in the manufacture of tents, outer garments, tarpaulins and the like, which comprises an outer spunbonded layer having ultraviolet radiation resistance imparted thereto, an intermediate microporous meltblown layer, preferably densified for resistance to liquid strike-through, and an inner spunbonded nonwoven layer treated for flame retardancy. The spunbonded layers preferably are formed with spotbonds, and have a basis weight of from about 0.5 to 5 ounces per square yard, with the intermediate meltblown layer having a basis weight of from about 0.5 to 2.0 ounces per square yard. Also disclosed is an embodiment wherein the spunbonded/meltblown/spunbonded laminate is pattern-bonded in a gross pattern occupying an area of 5 to 20% of the surface at a bond density of about 10 to 40 bonds per square inch. As shown in FIG. 4 of the patent, the laminate is spotbonded, and then pattern-bonded with a gross pattern of surface depressions being applied to both sides of the laminate; each layer of the laminate is correspondingly deformed by the gross pattern-bonding. The laminate is formed by lay-down of a spunbonded layer on a support belt, with the meltblown layer being formed directly on the spunbonded web, and a second spunbonded web then being applied to the meltblown layer to complete the composite, following which the entire composite is passed through a nip roll assembly for pattern-bonding. The laminate formed by the method of this patent suffers the same deficiency as the embossed web in the previously described Sciaraffa, et al. patent, in that the entire laminate is bonded, the spunbonded layers being dually bonded. Accordingly, the laminate by the inherent character of the pattern-bonding process has reduced flexibility and surface softness characteristics which, although not severely detrimental in the uses contemplated in the Bornslaeger patent, viz, in tents, outer garments, tarpaulins, and the like, render the laminate inadequate for the end-use applications contemplated for the present invention.
U.S. Pat. No. 3,912,567 to R. J. Schwartz discloses a process for intermittent autogenous bonding of a continuous filament web. The web is passed directly through a nip formed by a smooth hard-surfaced roll and a roll containing raised points on its surface, both rolls being maintained at a temperature near the softening point of the filaments. This process is carried out such that the temperature of the web is not substantially increased before maximum pressure has been developed in the nip, but at maximum pressure is rapidly raised to effect surface fusion before a significant increase in filament crystallinity occurs. The purpose of the disclosed process is to provide two-sided surface abrasion resistance, with good physical strength properties, for high basis weight webs. The term "intermittent autogenous bonding" in this patent refers to bonding by application of heat to a substantially unbonded web at intermittent areas which define the upper and lower surfaces of intermittent regions of the web which are compressed under a pressure of at least about 2,000 psi. The process disclosed in this patent involves a two-sided, monolayer web, and utilizes only one embossing step.
U.S. Pat. No. 4,069,078 to M. D. Marder, et al. discloses a finishing process for preparing nonwoven bonded sheets having high delamination strength and uniform appearance. The starting lightly consolidated sheet material is embossed by passage through a nip formed between two rolls, one of which has a multiplicity of bosses on substantially its entire surface, the bosses having a height of about 50-100% of the thickness of the sheet, with tips which have at least one dimension less than about 0.64 cm and the most prominent of which, in aggregate, form an area which is from 1-50% of the area of the surface of the roll. The resulting embossed sheet is passed through a heating zone for fusion of the surface film-fibrils and then cooled below its distortion temperature, such heating/cooling steps being carried out for each of the two sides of the sheet to obtain a bonded sheet of suitable opacity. As in the previously described prior art, this patent discloses a process for a single nonwoven sheet of material, wherein a single embossing step is carried out.
U.S. Pat. No. 4,041,203 to R. J. Brock, et al. discloses a nonwoven material comprising an impregnated mat of thermoplastic polymeric microfibers, and a web of substantially continuous, randomly deposited, molecularly oriented filaments of the thermoplastic polymer. The microfiber mat and the continuous filament web are attached by autogenous bonding at intermittent discrete regions to utilize the continuous filament web as a load bearing constituent of the material which has desired strength characteristics and possesses a textile-like appearance, drape and hand. In manufacture, the continuous filament web is formed by laydown of spun filaments on a foraminous carrier belt and the integrated microfiber mat is brought into laminar contact with the continuous filament web to form the unbonded two-ply laminate. Subsequently, the bonding attachment between the mat and web is effected by passage of the composite laminate through a pressure nip formed between heated rolls, one of which contains a plurality of raised points on its surface. An intermittent bond pattern preferably is employed, so that the area of the web occupied by bonds after passage through the nip is about 5-50% of the surface area of the materials, the discrete bonds being present at a density of about 50-1000/in.sup.2. This patent discloses a multilayered web, but the layers are bonded by only a single thermal embossing step, so that it suffers the disadvantages referred to hereinabove, viz, loss of flexibility and soft surface texture.
U.S. Pat. No. 4,436,780 to H. W. Hotchkiss, et al. discloses a nonwoven wiper laminate including a relatively high basis weight intermediate layer of meltblown thermoplastic microfibers, e.g., of polypropylene, and outer lightweight layers of generally continuous filament thermoplastic fibers, e.g., spunbonded polypropylene, having a larger average diameter. In the manufacture of the disclosed laminate, the respective layers are superpositioned relative to one another and the tri-plied composite then is passed through the nip between a patterned roll and anvil roll to pattern bond same. Again, this patent discloses a multilayer composite wherein a single embossing step is utilized for the composite.
U.S. Pat. No. 3,949,130 to R. M. Sabee, et al. discloses a spunbonded web of continuous synthetic filaments having one side that is at least two times smoother than the opposite side, wherein the majority of filament cross-points within the web are fuse-bonded to one another during the spinning of the web. The laydown of the filaments on a collection surface results in flattening on the laydown side to produce a smooth surface, the other side of the web comprising filaments which are randomly entangled to form a rough surface. Such web is disclosed to be useful in disposable diaper or like articles in which the rough side of the web faces and serves to anchor an absorbent pad, preferably also acting as a moisture carrier for wicking moisture through the web and into the absorbent pad, and the smooth side of the web provides a surface for comfortable contact with the baby's skin.
As shown in FIG. 6 of this patent, there is a steep density gradient from the smooth side to the rough side of the disclosed web, the density for the smooth side being approximately 0.32 gm/cm.sup.3 and the density of the rough side being approximately 0.04 gm/cm.sup.3. Thus, the smooth side of the web is of higher density which increases the difficulty of liquid penetrating into the web, in contradistinction to the rough side which is of lower density and, as mentioned in the patent's Abstract, has utility for wicking moisture through the web and into the absorbent pad. The disclosed web has texture characteristics on the respective sides which are appropriate for the intended use, i.e., a smooth side against the baby's skin and a rough side which serves to prevent shifting or displacement of the absorbent pad disposed contiguous thereto, but such textural characteristics are at odds with the function of the web in providing fast and intensive wicking action for removal of liquid from contact with the baby's skin. Accordingly, it would be appropriate if the density characteristics of the respective smooth and rough sides were reversed relative to that shown in FIG. 6, with the smooth side adjacent the baby's skin having a lower density and the rough side having a higher density thereby enhancing the anchoring action of the rough side while providing a low density, high loft fluffy baby-side surface.
U.S. Pat. No. 4,377,615 to M. Suzuki, et al. discloses a nonwoven fabric comprising an upper layer having a substantially smooth surface and a lower layer having a density lower than that of the upper layer. The upper layer comprises hydrophobic fibers as a principal element, the denier of which is finer than the denier of the lower layer, and contains a larger amount of adhesive bonding materials than the lower layer. The lower layer comprises hydrophilic fibers and hydrophobic fibers, the denier of which is coarser than the denier of the upper layer, and contains a smaller amount of adhesive bonding materials than the upper layer.
The Suzuki, et al. patent states that the upper and lower layers do not indicate a state wherein the thickness of the nonwoven fabric is equally divided into two but rather a case wherein a state of a plurality of fibrous webs formed through mixing of different fibers are overlapped to constitute a nonwoven fabric. The nonwoven fabric in such case is divided into an upper layer having a relatively higher density and a lower layer having a relatively lower density, density referring to the amount of fibers and adhesive bonding materials in each of the upper and lower layers, being averaged. The patent discloses to use fibers of polyester, polypropylene, acrylic, rayon, acetate and the like for each of the respective layers. The adhesive bonding materials described in this patent include those comprising as a main component acrylic ester copolymers, consisting of monomers such as ethylacrylate, methylacrylate and/or butylacrylate, wherein ethylacrylate is a major component.
In the manufacture of the nonwoven fabric disclosed in the Suzuki, et al. patent, the fibers for the respective layers are prepared, formed into webs and piled up by a plurality of cards. The resulting web then is guided to a saturator, where the web is dipped in a low-solids binder emulsion. The amount of binder applied to the lower layer by the saturator is comparatively small with respect to the upper layer, which downstream of the dip zone is sprayed with a higher-solids binder emulsion, whereby the upper layer has a greater binder content than the lower layer. Subsequently, the web is passed through serial driers, and then guided into contact of its upper higher density layer with a smooth-surfaced cylinder, where the web is forcibly pressed against the cylinder's surface by a plurality of rolls, to cure the web and provide a substantially smooth surface on its upper surface.
In the specification of the Suzuki, et al. patent, at column 5, line 45 to column 6, line 21 thereof, the preparation of various sample webs according to the disclosed invention is described, wherein the respective layers are formed and "these webs are piled." There is no disclosure of any type of bonding of respective layers in the web to one another; contrariwise, the webs are merely piled relative to one another, so that there is only a mechanical entanglement of fibers therebetween at the interface of the two layers. Although a bonding medium subsequently is applied to the respective top and bottom surfaces of the composite web, it would be expected that consistent with the teachings of the patent, there is no flow-through or penetration from one layer to another, since same would destroy the density gradation which is stated to be an object of the fibrous web according to the patent, i.e., each of the respective layers having its own specific density as associated with the extent of the bonding medium applied thereto. Thus, the interface will be defined by a comparatively loose assemblage of fibers which have a low level of structural integrity relative to one another so that constituent layers of the web may shift relative to one another in use. Further, the fibrous web described by this patent has a significant deficiency in that a substantially smooth surface is provided on the outer surface of the higher-density, more extensively bonded layer. Accordingly, the smooth surface in operation will function to oppose wicking or penetration of liquid through the laminated web to the contiguously positioned absorbent pad (disposed against its smooth surface). In other words, while the fluffy back surface of the fibrous web of this patent is effective to sorb fluid from a baby's skin, there is presented to such sorbed fluid a transmission barrier in the form of the substantially smooth surface positioned between such fibrous web and the absorbent pad.
U.S. Pat. No. 4,013,816 to R. N. Sabee, et al. discloses a stretchable spunbonded web suitable as a top-liner for a disposable diaper, pad, bandage and the like. The web is formed of a polyolefin and crossover points of the fibers do not rupture when the web is stretched to 50% of its original length, but maintains approximately its original structure. The web is made by melt-blowing polypropylene of less than 1.2 intrinsic velocity at a filament velocity at least 15 meters per second and at a denier per filament of 3, with webs being collected on a rotating chilled drum at 40.degree.-65.degree. F.
In U.S. Pat. No. 3,457,338 to L. E. Lefevre, there is disclosed a process for forming crimped polyolefin filaments of improved bulkiness, wherein the polyolefin material is extruded, at a melting temperature usually above about 130.degree. C., through a multihole die or spinneret to form a multiple filament strand. The multifilament strand is surface chilled immediately after extrusion by subjecting the filaments to a transverse flow of an inert cooling medium, such as air or nitrogen, against their outer peripheries at a temperature less than the melting temperature, generally in the range of 5.degree.-25.degree. C. The coolant impinges on all of the hot filaments at the same predetermined distance from the extruding die while not forcing them into coherence with one another. The chilled filaments then are oriented and stretched to 1-4 times their original length, subsequently passing through a first heating zone wherein the filaments are twisted about their longitudinal axis in one direction at a temperature of about 100.degree.-225.degree. C. and then a second heating zone wherein the filaments are twisted about their longitudinal axis in a direction opposite to that in the first zone, at a temperature of from about 100.degree.-225.degree. C.
U.S. Pat. No. 3,480,709 to I. Jacob, et al. discloses the production of filaments of high molecular weight synthetic linear polymers with a three-dimensional crimp, formed from melt-spun filaments which are cooled rapidly on one side below the spinneret on a cylindrical or flat cooling body at a temperature of from 0.degree.-70.degree. C. After contact with the cooling body, the crimp of the filament is at first invisible and is imparted to the filament in a latent form, as evidenced by the fact that the individual filaments after leaving the cooling body can be collected without sticking together. When filaments with such latent crimp are drawn at room temperature, followed by a subsequent tension-free heat treatment at a temperature of from 70.degree.-230.degree. C., a helical three-dimensional crimp is created. The patent states at column 4, lines 22 et seq that the filaments may first be cut and the crimp thereafter developed either on the fibers or on the finished yarn, woven or knitted fabric or fleece.
U.S. Pat. No. 3,577,498 to T. Matsuo, et al. teaches to form bulky crimped polypropylene fibers by asymmetrically cooling or quenching the polypropylene filaments in the cross-sectional dimension thereof to create cross-sectional anisotropy, followed by stretching the filaments to a draw rate of at least 2.5 times and then heat treating same in a relaxed state at a temperature of 80.degree.-150.degree. C. to develop coil-like crimps in the fibers.
U.S. Pat. No. 3,920,784 to J. Nakhgawa, et al. discloses the production of crimped fibers from asymmetrically cooled filaments which are partly non-circular in cross-section. The disclosed cross-sectional shapes consist of a substantially circular basic part and two or three projections therefrom. The patent further discloses to enhance uniformity of crimps by subjecting the disclosed filaments after asymmetrical cooling to drawing and mechanical crimping, to provide 5 to 15 crimps per inch. The crimped filaments then are spread in the form of a tow to reduce their apparent density to less than 0.15 gm/cc and the tow is heated under zero tension conditions.
U.S. Pat. No. 4,159,297 to J. K. P. Mackie, et al. discloses forming crimped filaments of semi-crystalline polyolefins or blends thereof with other materials, by melt drawing filaments at a drawing rate of less than 100 meters per minute, followed by rapid and asymmetric cooling of at least portions of the filaments, which then are formed into a tow and subjected to heat treatment, without drawing, at a temperature of at least 100.degree. C. The filaments thereafter are drawn in two stages, the last of which is at a temperature of at least 70.degree. C., then relaxed and subjected to a further, crimp-developing heat treatment, which may be applied either to the filaments themselves or to products produced therefrom.
U.S. Pat. No. 4,346,052 to J. R. Knox discloses a process for forming homogeneous curly synthetic polymer fibers, wherein after melt spinning of fibers from a slow crystallizing synthetic polymer composition, longitudinal tensile forces are applied to the fibers above the crystallization temperature range during a controlled substantially axially symmetric cooling of the fiber. The fibers thus formed have a substantially axially symmetric, residual tensile force differential between their outer sheaths and inner portions, being generally of helical configuration with more than about 15 turns per linear centimeter.